A turbocharger converts waste energy in the exhaust gas of an automotive engine into compressed air, which is then forced back into the automotive engine. This results in the engine burning more fuel and thus producing more power, while less energy is consumed, thereby improving the overall efficiency of the combustion process. A turbocharger typically comprises a turbine wheel and a compressor wheel, which are connected by a common shaft supported on a bearing system. The turbine wheel is driven by the exhaust gas which in turn drives the compressor wheel, the compressor wheel drawing in and compressing ambient air which is then fed into the engine's cylinders. By means of turbocharging the performance level of smaller engines can be increased up to the performance level of bigger engines without turbocharging, with lower fuel consumption and emissions. Consequently, turbochargers are increasingly employed with diesel and gasoline engines in passenger, commercial, off-road and sport vehicles.
Determining rotational speed of the compressor wheel of a turbocharger is important for optimizing its efficiency, and for ensuring that a turbocharger and engine stay within their respective safe operational ranges. Today's turbochargers need to operate reliably and continuously with increasingly higher exhaust gas temperatures and compressor inlet temperatures. Compared with its diesel counterpart, a modern gasoline turbocharger has to operate, in a much higher underhood temperature environment, with temperatures at the compressor wheel being around 200° C. or above. Modern turbocharger compressor wheels are typically constructed from strong, lightweight conductive materials such as aluminum, titanium or magnesium which can tolerate high stresses. Rotational speed of such compressor wheels can be measured, preferably by means of an active eddy current principle, wherein a magnetic field is generated by an oscillating system and a sensing coil is used to detect compressor blades when they pass through the magnetic field in front of the sensor tip.
Electronic circuits, components and interconnections which are commonly used in automotive applications are typically unable to withstand continuous exposure to temperatures above 150° C. Consequently, turbocharger speed sensors are often realized by connecting the sensor head/sensor tip with the sensing element that is located close to the compressor wheel via a cable to the sensor electronics, which are located in a separate housing at a distance from the turbocharger in a cooler environment within the engine compartment.
However, the separation of sensing element and sensor electronics may lead to higher vulnerability with respect to disturbances from other electronic devices used within the automobile, larger space requirements, higher instability, lower overall performance, decreased signal quality, and higher manufacturing costs. Also packaging, housing, and sealing are duplicated, once for the sensing element and once for the sensor electronics. I.e., manufacturing complexity is increased.
Often sensor electronics are placed inside the same part of the housing segment which is used for connecting the speed sensor to an on-board electronic network of the automobile and/or an engine control unit, which might however have a strong negative impact on signal quality. Also, depending on the particular application the separately located sensor electronics might be increasingly subjected to vibrations, shocks or similar, which might damage the sensor electronics and/or cause their malfunctioning.
Nowadays applications for measurement of turbocharger speed are challenging in that impeller/compressor wheels (the target wheels) are typically very thin (a few tenths of a millimeter; in particular for passenger cars) and therefore deliver a low signal. Also the sensing distance/air gap, i.e., the distance between the sensing element (typically a standard flat coil such as a pancake coil) and the target (the blades), varies as the coil is flat while the interior wall of the turbocharger housing is round/saddle-shaped and the envelope of the impeller/compressor wheel is curved.
Currently, relatively large coils are employed to obtain a strong enough output signal. However, in turbo charger applications there is a need for small sensing elements/coils to permit a small/thin sensor device tip to avoid negative side effects such as hot spots and aerodynamic disturbances which may have a negative impact on the turbocharger's functioning.
With a large flat coil disturbing effects of varying sensing distance/air gap are even stronger, and forming such large coils into a saddle shape is technically difficult. Furthermore, sensor tips with such a large coil are too big to be inserted into small turbochargers of the new generation. The coil is usually connected to the sensor device by welding which, however, is a complex and critical production step.
In DE 10 2009 027 853 A1 the speed sensor is arranged in a recess in the wall of the turbocharger housing with the sensing tip pointing toward the compressor wheel. However, the sensor tip is separated from the inside of the turbocharger by a heat-resistant element. Furthermore, the sensor electronics are located outside of the turbocharger.
Similarly, WO 2013/102510 A1 discloses a speed sensor arranged in the wall of a turbocharger with the sensor tip being separated from the inside of the turbocharger (i.e. the compressor wheel cavity) by a thin wail and a gap between the thin wall and the sensor tip, i.e. the sensor tip is located in a closed or “blind” hole spaced apart from the bottom wail of the blind hole. Electronics such as an application-specific integrated circuit (ASIC) are located in the flange of the sensor housing that is. mounted onto the outside wall of the turbocharger housing. Hence, in both cases the speed sensors need to be separated by a thin wall/a heat-resistant element from the inside of the turbocharger, which may however impair signal quality. Furthermore, the sensor electronics are arranged outside of the turbocharger housing.
U.S. Pat. No. 7,378,721 B2 discloses a speed sensor for a turbocharger, the speed sensor having a cylindrical housing for incorporation into a turbocharger wall. Inside the cylindrical housing a lead frame substrate is arranged that supports at least one electronic component. The electronic component is connected with wire bonds to the lead frame substrate, with the lead frame being encapsulated by a thermoset plastic. Conductive epoxy is used to maintain a connection between the electronic component and the lead frame, The need for a printed circuit board or a ceramic substrate is avoided by using the lead frame substrate. The electronic component may comprise an application-specific integrated circuit (ASIC). The speed sensor can withstand temperatures of approximately 190° C. at the compressor side of the turbocharger.